It is known to use voice frequency keypads that operate by pressure on a key causing two frequencies to be emitted simultaneously. Generally they are designed to comply with Notice No. Q23 of CCITT. However, in recent prior art apparatus, digital technology methods have been used to generate these frequencies and perform the various switching operations. Digital technologies are subject to errors during transmission which are due to the bivalency of their all or nothing states.
The keypad in accordance with the invention remedies this drawback. Indeed, the method used therein is entirely analog.